A. Field of the Invention
The present invention relates to a gallium nitride semiconductor device and a method for producing the same.
B. Description of the Related Art
A gallium nitride (GaN) compound semiconductor (hereinafter referred to as GaN semiconductor element) has been heretofore used as a semiconductor material of a semiconductor element for high-frequency device use. In the GaN semiconductor element, a buffer layer and a doped GaN layer, for example, formed by a metal organic chemical vapor deposition (MOCVD) method are provided on a surface of a semiconductor substrate.
FIG. 14 is a sectional view showing a lateral structure gallium nitride semiconductor element according to the background art. As shown in FIG. 14, in the lateral structure GaN semiconductor element, a buffer layer 1002, silicon-doped n-type GaN layer 1003 and aluminum gallium nitride (AlGaN) layer 1004 are laminated in this order on a surface of semiconductor substrate 1001. Buffer layer 1002 corresponds to a raw material used for the semiconductor substrate 1001. Surface barrier layer 1005 formed by a two-dimensional electron gas (2DEG) is provided in an interface between n-type GaN layer 1003 and AlGaN layer 1004. Surface barrier layer 1005 serves as a conductive layer exhibiting high channel mobility. Source electrode 1012, drain electrode 1013, and gate electrode 1016 which is provided with interposition of insulating film 1007 are formed on a surface of AlGaN layer 1004. Although almost all lateral structure GaN semiconductor elements have the structure as shown in FIG. 14, various GaN semiconductor elements having high voltage withstanding have been achieved as other type GaN semiconductor elements by formation of a structure for achieving high voltage withstanding and a structure of a gate electrode.
In this GaN semiconductor element, a voltage applied to gate electrode 1016 is regulated to control the electron concentration of surface barrier layer 1005 to thereby turn on/off a current flowing between the source and the drain. Generally, in such a GaN semiconductor element, connection between the source and the drain is electrically conductive (hereinafter referred to as ‘normally-on’) when there is no voltage applied to gate electrode 1016. Therefore, a configuration in which a gate electrode having Schottky characteristic is formed as gate electrode 1016 or a configuration in which a p-type layer is formed between AlGaN layer 1004 and gate electrode 1016 is used to control the electron concentration of surface barrier layer 1005.
The normally-on type GaN semiconductor element however has limited uses. Therefore, a GaN semiconductor element in which connection between the source and the drain is not conductive (hereinafter referred to as ‘normally-off’) when there is no voltage applied to gate electrode 1016 has been proposed recently. A GaN semiconductor element having a metal oxide semiconductor field effect transistor (MOSFET) structure is an example of the normally-off type GaN semiconductor element.
Lateral structure transistors with a withstand voltage achieved in a range of from several hundreds of V to several kV at maximum have been produced by way of trial as high withstand voltage devices using such GaN semiconductor elements. Some of these lateral structure transistors have been commercially available. When a transistor using a GaN semiconductor element is used as a switching device of a power converter such as an inverter, on-resistance of the GaN semiconductor element can be reduced compared with that of the Si semiconductor element according to the background art, and the switching device can be operated speedily. Accordingly, reduction in size of the semiconductor element can be attained. Moreover, power loss can be greatly reduced. For this reason, both reduction in size and increase in power density of the switching device can be achieved. Moreover, because the GaN semiconductor element can be used at a high temperature compared with the Si semiconductor element, there is an increasing demand that the GaN semiconductor element should be used in a high temperature environment, such as in the area around a car engine.
The following device has been proposed as a semiconductor device using such a GaN semiconductor element. While a SiCMOS switching transistor is formed on a Si-off substrate, an AlGaN—GaN field effect transistor is formed so as to be integrated with the SiCMOS switching transistor through a GaN buffer layer. A voltage is applied to at least one terminal of the AlGaN—GaN field effect transistor by a DC-DC converter. Part of the DC-DC converter is formed of a SiCMOS switching transistor (see, for example, JP-A-2004-281454).
The following device also has been proposed as a variation. In a GaN semiconductor integrated circuit in which a plurality of GaN semiconductor elements which differ in kind are integrated on one substrate, the GaN semiconductor elements include a Schottky diode and a field effect transistor. A first anode electrode is provided on a GaN semiconductor layer having a predetermined width and forming the Schottky diode so that the first anode electrode is Schottky-contacted to the GaN semiconductor layer with a narrower width than the predetermined width. A second anode electrode is provided on another portion of the GaN semiconductor layer than the portion contacted to the first anode electrode so that the second anode electrode is Schottky-contacted to the GaN semiconductor layer and electrically connected to the first anode electrode. The height of a Schottky barrier formed between the first anode electrode and the GaN semiconductor layer is lower than the height of a Schottky barrier formed between the second anode electrode and the GaN semiconductor layer (see, for example, JP-A-2006-100645).
The following device has been proposed as another device. A GaN layer laminated on a substrate directly or through a buffer layer, a plurality of transistors formed near a surface of the GaN layer, an oxide or nitride film for covering front and side surfaces of the transistors, and an AlGaN layer laminated on the oxide or nitride film-including GaN layer by ELO are formed by repeated lamination in accordance with the number of the transistors to be integrated (see, for example, JP-A-2008-198675).
The following device has been proposed as yet another device. That is, a device includes a conducting layer, a channel layer of a Group III-Group V nitride semiconductor formed above the conducting layer, a Schottky layer of a Group III-Group V nitride semiconductor formed on the channel layer, a first source electrode, a drain electrode and a gate electrode formed in part above the Schottky layer, a second source electrode connected to the first source electrode, and a wiring member for connecting the first source electrode and the conducting layer to each other through a groove piercing the channel layer and the Schottky layer (see, for example, JP-A-2006-086398).
The following device has been proposed as another device. That is, a device includes a substrate, a nitride semiconductor layer formed on a principal surface of the substrate and having a channel region in which electrons run in a direction parallel to the principal surface, a plurality of first electrodes and a plurality of second electrodes formed so as to be separated from one another and disposed alternately on an active region of the nitride semiconductor layer, a first insulating film and an interlayer insulating film formed in ascending order on the nitride semiconductor layer and having a plurality of openings in which the first electrodes are exposed respectively, and a first electrode pad formed on a region of the interlayer insulating film above the active region and electrically connected to respective portions of the first electrodes exposed from the openings. The substrate further has a second electrode pad which has conducting property and which is formed on an opposite surface to the principal surface of the substrate. The second electrode pad is electrically connected to the second electrode (see, for example, JP-A-2008-177527).
When the aforementioned high withstand voltage GaN semiconductor element is used as a switching device (hereinafter referred to as ‘semiconductor switch’), a semiconductor device is often configured so that semiconductor elements are operated alternately by use of a plurality of semiconductor switches at predetermined timing. FIG. 15 is a circuit diagram showing a semiconductor device using semiconductor switches according to the background art. The switching circuit shown in FIG. 15 includes semiconductor switches 1040 to 1042 (fourth semiconductor switch et seq. are not shown), control circuit 1048, ground terminal (hereinafter referred to as ‘GND terminal’) 1046, control signal input terminal (hereinafter referred to as ‘IN terminal’) 1047, circuit portion power supply terminal (hereinafter referred to as ‘VD terminal’) 1049, and high withstand voltage output terminals (hereinafter referred to as ‘OUT terminals’) 1050 to 1052. The semiconductor switches 1040 to 1042 and control circuit 1048 are formed in semiconductor devices 1100 to 1300 and semiconductor device 1400 respectively.
Control circuit 1048 is a circuit for driving semiconductor switches 1040 to 1042 alternately at predetermined timing. An input signal from IN terminal 1047 is fed as one of gate input signals 1043 to 1045 to one of semiconductor switches 1040 to 1042. When, for example, semiconductor switch 1040 should be driven, gate input signal 1043 is selected.
In this situation, it is necessary to dispose semiconductor switches 1040 to 1042 as near as possible. However, because the semiconductor switches are formed separately in semiconductor devices 1100 to 1300 respectively, the distance between adjacent ones of the semiconductor switches is limited, for example, by the size of a mount region or each semiconductor device in a printed substrate. Moreover, it is necessary to provide connection pad regions, external wiring regions, etc. for connecting the respective semiconductor switches and control circuit 1048 to the outside. For this reason, it is difficult to reduce the total size of the switching circuit even if the GaN semiconductor element were used as a semiconductor switch. Moreover, there is a problem of signal delay due to external wiring or malfunction caused by production of noise. When a Si semiconductor element is used as an element forming control circuit 1048, the effect based on use of the GaN semiconductor element cannot be realized because control circuit 1048 cannot be used at a temperature exceeding about 200° C., which is a heat-resistant temperature of the Si semiconductor element.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.